1. Field of the Invention
The present invention relates generally to substrate surface cleaning and, more particularly, to a method and apparatus for improving high frequency acoustic energy cleaning of a semiconductor substrate following fabrication processes.
2. Description of the Related Art
As is well known, megasonic cleaning is widely used in semiconductor manufacturing operations and can be implemented in a batch cleaning process or a single wafer cleaning process. In a batch cleaning process, the vibrations of a megasonic transducer creates acoustic pressure waves in the liquid medium of a cleaning tank containing a plurality of semiconductor substrates. Normally, in megasonic cleaning of multiple batches of semiconductor substrates in the cleaning tank, the semiconductor substrates are static (i.e., stationary) allowing multiple reflections of the acoustic energy to be averaged, using the design of the tank and placement of the wafer cassette to minimize energy ‘dead zones’ or energy ‘hot spots’. Hot spots (i.e., high energy regions) are caused due to constructive interference of megasonic wave reflections from both the multiple wafers and the megasonic tank walls while cold spots (i.e., low energy regions) are caused due to destructive interference of same.
In a single wafer megasonic cleaner, however, a small transducer is defined above a rotating wafer, wherein the transducer scans across the rotating wafer using a fluid meniscus coupling. Alternatively, in the case of full immersion of the semiconductor wafer in a single wafer tank system, the acoustic energy is typically transmitted to and through the liquid medium to the semiconductor wafer.
FIG. 1A is a simplified top view of a megasonic transducer 10, in accordance with the prior art. The megasonic transducer is fabricated using a plurality of crystals 14a–14d of piezoelectric material bonded to a resonator 12. The crystals 14a–14d are shown to be bonded to the resonator such that a gap exists between each pair of adjacent crystals. The acoustic energy imparted by the transducer 10 is averaged as a result of the rotation of the semiconductor substrate about the transducer 10.
The performance of the transducer is determined by the material properties of the piezoelectric crystals as well as the bonding method of the crystals 14a–14d to the resonator 12. Currently, high and low energy zones are created radially across the semiconductor substrate during the megasonic cleaning, resulting in variations in cleaning efficiency as well as radially dependent damage across the semiconductor substrate if the peaks in energy are above the damage threshold.
One of the primary causes of variation in cleaning efficiency is the existence of the gap regions 46 defined between each pair of adjacent crystals 14a–14d. Specifically, each gap region 46 creates a zero-energy zone, which in turn, forms a band of defects at a specific radius of the semiconductor wafer. The bands of defects each corresponding to a gap region 46 is one of the primary sources of having non-or minimal cleaning in the zero energy zones.
Creation of bands of defects at specific radii is shown in FIG. 1B, in accordance with the prior art. Gap regions 46a and 46b are shown to have been respectively defined between adjacent crystals 14a–14b and 14b–14c. The dead energy zones corresponding to the gap regions 46a and 46b are shown in the average energy versus distance plot, shown in FIG. 1C of the prior art. As can be seen, the high energy zones 16c–16a correspond to centers 16a–16c of the crystals 14a–14c, respectively. While, the dead energy zones 46a–46b respectively correspond to the gap regions 46a and 46b. The non-uniform cleaning of the semiconductor substrates resulting from dead-zone banding effect undesirably results in production of defective semiconductor substrates.
One way to avoid the dead zone banding effects generated by array of small crystals is implementing a single piezoelectric crystal 22 bonded to the resonator 12, as shown in FIG. 1D of the prior art. Although implementing the single crystal transducers is beneficial in eliminating the bands of defects, attempting to uniformly bond the single crystal 22 to the resonator 12 is very difficult and challenging. As can be seen, voids 22a–22c are created between the single crystal 22 and the resonator 12 during the bonding, negatively affecting the performance of the transducer and resulting in non-uniform cleaning. Additionally, bonding the single piezoelectric crystal 22 to the resonator 12 is more costly than bonding a plurality of small crystals.
In view of the foregoing, a need therefore exists in the art for a single wafer cleaning system capable of uniformly distributing acoustic energy on semiconductor substrates being cleaned at a lower cost, while substantially eliminating damaging dead zone band effects.